If you know how it works between two parties then you can just scale it up to multiple recipients. The important thing here is that you have to encrypt messages individually.

There are of course multiple possibilities of how to achieve this, here are two simplified examples:

Asymmetric encryption

1. Import John‘s, Mike‘s and Dave‘s public key
2. Encrypt the message with their public key individually (don‘t encrypt a message twice)
3. Send the encrypted messages to the recipients
4. The recipients (John, Mike & Dave) can decrypt the message by using their own private key

Symmetric encryption

1. Create a symmetric key, i.e. for AES
2. Encrypt the message with this key
3. Send the encrypted message to the recepients
4. Encrypt the symmetric key with the recipients‘ public keys individually
5. Send the encrypted key to the recipients
6. The recipients can decrypt the symmetric key by using their own private key
7. The recipients can now decrypt the message with the symmetric key